Heating boilers of the above-mentioned type are known from German Offenlegungsschrift No. 23 13 186, especially with regard to the sleeve in the region of the flue-gas collecting chamber.
The combustion gases have, as is known, the lowest temperature at the boiler end. If the temperature of the boiler wall lies below 65.degree. C., the combustion gases condense upon combustion of oil or gas. With oil, an acid condensate forms, consisting of sulphurous acid or sulphuric acid which strongly attacks the iron materials and finally destroys them in course of time. Owing to the expensiveness of heating energy, it is necessary to save heating energy. This is to be achieved, among other things, by heating the boiler only when heat is required; that should be, the boiler is operated not at constant and elevated temperature, but at so-called sliding temperature and can cool off from time to time. If the boiler is not protected sufficiently against corrosion by special measures, acid condensate is formed upon reheating and there is consequently a risk of corrosive attack.
It is most appropriate to make the heating boiler dry, or in other words, to design as large a part as possible of the heating faces in such a way that also at low boiler water temperature they assume, immediately after the burner is switched on, a temperature which lies above the dew-point temperature of the combustion gases. Combustion gases can then not condensate at all and cause damage.
With the known heating boiler, this is achieved by making the heating boiler cylindrical, U-shaped profiles being welded on the inside as fuel-gas flues. These U-shaped profiles assume, immediately after the oil burner has been switched on, a higher temperature or a sufficiently high temperature to prevent combustion gases from condensing. In front of the flues in the region of the guide chamber, the radiant heat of the flame is so high that the combustion gases do no condense or evaporate again immediately. Behind the flues in the region of the collecting chamber, the arrangement of a sleeve preventing heat transmission has been successful, but it has proved necessary to improve the sleeve with regard to manufacturing costs.